Conventional papermaking machinery for producing a continuous sheet of paper includes equipment to set the sheet properties of the paper as the sheet of paper is being manufactured. Generally, on-line measurements of sheet properties, such as thickness, moisture, gloss, or smoothness are made by scanning sensors that travel back and forth across the width of the sheet of paper in the cross-machine direction (CD). The scanning sensors are located downstream of actuators that are controlled to adjust the sheet properties. The scanning sensors collect information about the sheet properties to develop a property profile across the sheet and provide control signals to the appropriate actuators to adjust the profile toward a desired target profile in a feedback loop. In practice, the actuators provide generally independent adjustment at adjacent cross-directional locations of the sheet, normally referred to as slices or profile zones.
Other configurations include an array of non-scanning sensors disposed across the width of the sheet that collect information about the sheet properties to develop a property profile across the sheet and, as described above, provide control signals to the appropriate actuators to adjust the profile toward a desired target profile in a feedback loop.
One of the more basic operations on a papermaking machine is control of the cross-direction moisture profile by heating the paper sheet in zones with a steambox that is typically located in a press section of the papermaking machine. Cross-direction moisture profiles can also be controlled by using re-wet actuators (e.g., water sprays administered by spray nozzles) typically located near the reel. By applying water to the drier areas of a sheet, a uniform CD moisture profile can be created.
In the manufacturing of a flat sheet of paper, the cross-machine direction uniformity is a critical issue. Current automation systems use measurements, control algorithms, and actuators to directly control the various sheet characteristics in multiple zones across the sheet. As an example, in the paper industry, it is common to have cross-machine direction actuators that add steam in various zones across the sheet to increase the temperature of the water, which facilitates better removal of the water in those zones, but usually results in reduced sheet bulk. Later in the sheet manufacturing process, it is common to spray water on the sheet in zones, which further affects the cross-machine direction moisture content of the sheet. However, adding water is not energy efficient and can result in greater variation in paper quality than if only the correct amount of water was removed from the sheet. Machine clothing is used to assist in transporting the sheet along the papermaking machine. Machine clothing also aids in the removal of water and other solvents from the sheet during the manufacturing process. For example, it is common to use machine clothing (i.e., press felts) to accompany the sheet through a press nip and function as a repository for the water that is mechanically removed from the sheet. As the sheet moves through the papermaking machine, the press felt re-circulates and is washed and dried and made ready to assist in transporting the sheet along the papermaking machine. All of the actions on the press felt are typically in the machine direction (MD).
Typically, actuation systems act directly on the sheet. However, actuators can act upon process equipment, which then, in turn, transfer an effect to the sheet, such as heating zones on calendar stacks or press zones in press rolls. Missing from the cross-machine direction control of flat sheets are actuators that act upon the cross-machine direction properties of the machine clothing, particularly press felts. Because the press felt directly affects the paper sheet quality, any cross-machine direction characteristics of the press felt that are affected are transferred to the sheet of paper.